Bioluminescence
Bioluminescence is the light that living things make inside their own bodies.
Think of it this way
A jar of clear liquid sits on a dark kitchen counter. Inside it, tiny glowing dots float near the bottom. The dots make their own light — they do not need a bulb or flame. Each dot glows because of a chemical change happening inside it.
Explaining bioluminescence by grade level
Fireflies glow at night to find a mate. They mix two things inside their body to make light. No heat comes from this glow. It is like a tiny cold lamp that turns on and off.